If you are not opposed to using chemicals you can apply glyphosate (Round Up) to kill the weeds. Once they are dead you can rototill the area. The dead weeds will end up being organic matter in the soil and except for weed or grass seeds you might bring up when you turn the soil over, the original weeds and grass should not be a problem in the future. After making your garden and planting it, mulch over the bare soil with 2-3 inches of compost or other organic matter and it will help suppress weeds. Good luck with your project!